Sliding wear of calcium α-sialon ceramics

Abstract

The sliding wear behaviour of calcium α-sialon ceramics having two distinct microstructures, large elongated-grained and fine equiaxed-grained was revealed by observing the evolution of wear rate, wear track and debris as a function of apparent contact pressure and sliding speed, using a ball-on-disc type wear tester. The thresholds for the transition from mild to severe wear in terms of apparent contact pressure and the controlling material removal mechanisms were identified for both microstructures. The large elongated-grained microstructure had a higher wear transition threshold and exhibited a reduced wear rate in the severe wear regime, compared to the fine equiaxed-grained microstructure. This behaviour is attributed to a greater resistance to crack extension arising from the large elongated-grained microstructure. As the apparent contact pressure decreased, mild wear occurred at pressures below the thresholds for each of the microstructures. The material removal for the large elongated-grained microstructure was controlled by transgranular fracture, whereas grain pull-out prevailed in the fine equiaxed-grained microstructure. Consequently, a greater wear rate results for the fine equiaxed-grained microstructure in mild wear regime. An increase in sliding speed caused a slight increase in wear rate for both microstructures, more evidently in the mild wear regime. Models were developed to clarify the effect of microstructure on severe and mild wear of Ca α-sialon. They revealed that grain aspect ratio plays a more important role than grain diameter in the control of wear behaviour.